1. Field of the Invention
The present invention relates generally to comb filter circuits and more particularly to a comb filter circuit suitable for use with a video tape recorder (VTR).
2. Description of the Prior Art
When a home VTR such as an 8 mm video tape recorder (8 mm VTR) reproduces a video signal, a cross talk component between adjacent tracks is usually generated in the luminance signal and in the carrier chrominance signal. In the following description, the cross talk component between the adjacent tracks will be referred to as the "cross talk component" for simplicity.
To solve this problem, during recording, the 8 mm VTR records the video signal in such a fashion that the luminance signal and the carrier chrominance signal are interleaved with each other between the odd-numbered field track and the even-numbered field track, or that the cross talk component is interleaved with a main signal. The main signal is what is reproduced from the original track. During reproduction, the cross talk components contained in the luminance signal and in the carrier chrominance signal cancel each other out in the comb filter.
FIG. 1 illustrates an example of a conventional comb type filter circuit for performing the above signal processing. In this example, a composite color TV signal (Y+C) is separated into a luminance signal Y and a carrier chrominance signal C.
Referring to FIG. 1, this comb type filter circuit generally comprises a comb filter section 10 formed of an integrated circuit (IC) and an integrated circuit (IC) section 20, to be explained in greater detail furtherherein, that includes a part of a processing circuit for processing the luminance signal Y and the carrier chrominance signal C.
In the comb filter circuit section 10, a CCD (charge-coupled device) delay circuit 11 is provided to have a delay time of 1H+.DELTA.D where 1H represents one horizontal period and .DELTA.D a predetermined number of bits of delay time. Also, CCD delay circuits 12 and 13 are provided, each of which has the same delay time of .DELTA.D. Accordingly, since the output signals from the delay circuit 11 and the delay circuit 12 (and 13) have a time difference of just one horizontal period (1H) therebetween, it is possible to form a comb filter circuit by adding or subtracting these output signals, as will be described later in greater detail. Although an adding circuit 14 and an attenuator 15 are included in the comb filter circuit section 10, they will also be described at a later stage.
The reason that the delay circuit 11 has the extra delay time of .DELTA.D and that the comb filter circuit section 10 includes the delay circuits 12 and 13 is as follows. Although the delay time of the CCD delay circuit can accurately be determined by a clock signal, the gain vs. temperature characteristic of the CCD delay circuit considerably fluctuates so that the fluctuating gain vs. temperature characteristic must be compensated for by some means.
To solve these problems, in the comb filter circuit section 10, signals, not delayed by the CCD delay circuit 11, are delayed by the CCD delay circuit 12 (and 13), whereby the fluctuating gain vs. temperature characteristic for each signal is compensated for. Further, since the CCD delay circuits 12 and 13 each delay the signal by the delay time of .DELTA.D, the delay time of the CCD delay circuit 11 is determined as 1H+.DELTA.D so that it can remove the time difference therebetween.
The IC section 20 comprises band-pass filters 22 and 38 each having a pass band of the carrier chrominance signal C, a switching circuit 24 that is connected in the opposite state to that shown in FIG. 1 when a dropout occurs and low-pass filters 31 to 33 for cancelling unwanted out-band components such as a clock component of the CCD delay circuit. In this example, each of the low-pass filters 31 to 33 has a delay time of .DELTA.D.
The IC section 20 further comprises a delay circuit 35 for the phase compensation, a trap circuit 37 for eliminating the band components of the carrier chrominance signal C, a limiter 41, an attenuator 42 and recording and reproducing change-over switching circuits 91 to 94. Each of the switching circuits 91 to 94 is connected to a contact R during recording and to a contact P during reproduction.
During recording, this filter circuit is connected in the illustrated state in FIG. 2, thereby separating from the composite color TV signal (Y+C) the luminance signal Y and the chrominance signal C. The switches 91 and 92, shown in FIG. 1, are set in their R positions and are omitted from FIG. 2 for the sake of simplicity.
As illustrated in FIG. 2, the incoming composite color TV signal (Y+C) is supplied through the delay circuit 12 to the adding circuit 14 and is also supplied through a phase inverting amplifier 21 and the delay circuit 11 to the adding circuit 14, forming a C-type comb filter 16. Thus, the adding circuit 14 produces a signal 2C having a level twice as a high as that of the carrier chrominance signal C.
Since the C-type comb filter 16 presents a C-type comb filter characteristic over the whole band of the composite color TV signal (Y+C) as shown in the frequency characteristic diagram of FIG. 3, the signal 2C from the adding circuit 14 is supplied to the attenuator 15, in which it is attenuated into the carrier chrominance signal C of the original level and is then supplied through the low-pass filter 32 to the band-pass filter 38, a subtracter 34 and the trap 37. Thus, the band-pass filter 38 produces only the original signal C. In FIG. 3, fc represents the color sub-carrier frequency and fh the horizontal frequency.
Turning back to FIG. 2, the composite color TV signal (Y+C) is supplied through the delay circuit 13 and the low-pass filter 31 to a subtracting circuit 34, and the signal C from the low-pass filter 32 is supplied to the subtracting circuit 34, forming a Y-type comb filter 17. Thus, the subtracting circuit 34 produces the luminance signal Y.
Since the Y-type comb filter 17 presents the Y-type comb filter characteristic for the whole band of the signal (Y+C), the luminance signal Y from the subtracting circuit 34 is supplied (through the switch 93 shown in FIG. 1) to the delay circuit 35 to compensate for the delay of the trap circuit 37, and then supplied to an adding circuit 36, while the signal C with the C-type comb filter characteristic for the whole band from the low-pass filter 32 is supplied through the trap circuit 37 to the adding circuit 36. Thus, the adding circuit 36 produces the signal Y which presents the Y-type comb filter characteristic only for the band of the signal C.
Then, the thus separated, predetermined format signals Y and C are recorded on a magnetic tape (not shown).
During reproduction, the above filter circuit (FIG. 1) is connected as shown in FIG. 4 whereby cross talk components Y.sub.X and C.sub.X are respectively removed from the reproduced luminance signal Y and carrier chrominance signal C.
Referring to FIG. 4, the reproduced carrier chrominance signal C with the cross talk component C.sub.X from the tape, that is, the signal (C+C.sub.X) is supplied through the band-pass filter 22 to the adding circuit 23. At the same time, the reproduced luminance signal Y with the cross talk component Y.sub.X from the tape, that is the signal (Y+Y.sub.X), is supplied through a normal side contact N of the switching circuit 24 to the adding circuit 23. Thus, the adding circuit 23 produces an added signal S expressed as (Y+Y.sub.X +C+C.sub.X). The signal S is supplied to the C-type comb filter 16 causing the low-pass filter 32 connected to it to produce the signal C and the cross talk component Y.sub.X (C +Y.sub.X). Then, the signal (C+Y.sub.X) is supplied to the band-pass filter 38 which produces the signal C.
Further, the signal (Y+Y.sub.X) from the switching circuit 24 is supplied through the delay circuit 13, the low-pass filter 31 and the delay circuit 35 to a subtracting circuit 39. Simultaneously, the signal (C+Y.sub.X) from the low-pass filter 32 is supplied to the trap circuit 37 which then produces the cross talk component Y.sub.X. This cross talk component Y.sub.X is supplied through the limiter 41 and the attenuator 42 to the subtracting circuit 39, forming a Y-type comb filter circuit 18. Thus, the subtracting circuit 39 produces the signal Y.
Since the cross talk component Y.sub.X is also a non-vertical correlation component and is linearly supplied to the limiter 41 and the attenuator 42, the filter 18 changes the depth (trough portion) of the curve of the out-band frequency in accordance with the cross talk component Y.sub.X.
The delay circuit 11 is provided with an intermediate terminal or tap from which a signal -Sc delayed by, for example, 1H-AD is derived. This signal -Sc is supplied to the low-pass filter 33 and is thereby delayed by the delay time, .DELTA.D. In other words, this signal -Sc is converted into a signal -Sd with a delay time of 1H by adding the delay time, 1H-.DELTA.D of the delay circuit 11 and the delay time of .DELTA.D. The signal -Sd is supplied to a phase inverting amplifier 25 and is thereby inverted in phase to be a signal Sd. This signal Sd is fed to the switching circuit 24. When a dropout occurs, the switching circuit 24 is connected to its compensating side contact C in response to a dropout detected output signal from a dropout detecting circuit (not shown), whereby the dropout is compensated for by the one horizontal period delayed signal Sd.
However, since the C-type comb filter 16 presents the C-type comb filter characteristic merely by subtracting the main signal and the one horizontal period delayed signal thereof, its pass band width .DELTA.f is broad as shown by a dashed line in FIG. 5. Thus, the C-type comb filter 16 cannot remove the cross talk component C.sub.X sufficiently.
To overcome the above shortcoming, it is proposed that the C-type comb filter be modified into a C-type comb filter 6 as shown in FIG. 6. This C-type comb filter 6 is formed as a feedback type that can improve its C-type comb filter characteristic.
Referring to FIG. 6, the C-type comb filter 6 comprises a one horizontal period delay circuit 1, a band-pass filter 2, subtracting circuits 3 and 4 and an adding circuit 5. The cross talk component C.sub.X is derived from the adding circuit 5 and this cross talk component C.sub.X is fed through the band-pass filter 2 back to the subtracting circuit 3.
Accordingly, since the C-type comb filter characteristic of the filter 6 has a narrow pass band width .DELTA.f as shown by a solid line in FIG. 5, its effect for cancelling the cross talk component C.sub.X out is improved over the C-type comb filter 16.
In this C-type comb filter 6, however, the delay time of the delay circuit 1 must be accurately selected to be one horizontal period. If the delay time of the delay circuit 1 is selected to be 1H+.DELTA.D just like the aforementioned CCD delay circuit 11, a time difference of .DELTA.D occurs between the input signal (C+C.sub.X) and the feedback signal C.sub.X so that the C-type comb filter 6 cannot present a C-type comb filter characteristic.
To present a C-type comb filter characteristic, the delay circuit 1 in the C-type comb filter 6 must be formed of a glass delay line like the prior art. Conventionally, a glass delay line, however, is disadvantageous from an assembly parts number and mountable space standpoint.